Isobutene intended for polymerization must have a purity level above 99% and it must only contain traces of but-1-ene and but-2-enes (a few hundred parts per million by weight, ppm). In fact, if the impurities of isobutene are too high, the resulting polymers are less good in quality and the polymerization yield is lower. As a result it is necessary to remove from a hydrocarbon cut containing isobutene other olefinic hydrocarbons containing 4 carbon atoms per molecule. Since but-1-ene and isobutene have very close boiling points, they cannot be separated by distillation, unless considerable means are used. The other olefinic hydrocarbons having 4 carbon atoms per molecule can be separated from isobutene by distillation.
The main problem that arises when producing high-purity isobutene is thus the separation of but-1-ene from isobutene. Several ways can be contemplated to carry out this separation.
The first way consists of sulfuric acid extraction: the isobutene is selectively hydrated and thereafter regenerated by treatment of the aqueous phase. If the temperature and the concentration are properly controlled, this process results in isobutene of good purity. However, the yield does not usually exceed 90% as extraction is not complete, and dimers and oligomers are formed, leading to the formation of toxic acid sludge.
The second way consists of cracking methyl tert-butyl ether (MTBE): the isobutene is extracted from the C.sub.4 cut by reaction with methanol in order to form MTBE. The MTBE is then cracked into methanol and isobutene on an acid catalyst. The recovery yield is generally 96%. The isobutene produced exhibits good purity but the dimethyl ether that may form during cracking has to be removed.
The third way that may be contemplated is the dehydration of tert-butyl alcohol (TBA). Methanol in the above process is replaced by water, and this leads to the formation of TBA. The isobutene is thereafter recovered by dehydration of the TBA. This way is not used commercially, mainly because TBA is very closely connected with the propylene oxide market. TBA can, depending on the processes, be a by-product of propylene oxide.
U.S. Pat. No. 5,177,283 describes a hydrocarbon conversion process comprising passing the feedstock into a fractionation zone, the top effluent being rich in one of the reactants and the bottom effluent being rich in reaction product, said process being such that a liquid flow is subjected to a side-stream draw-off, passed with a hydrogen-rich gaseous flow into a catalytic reaction zone, which gives rise to a reaction zone effluent comprising one of the reactants and the reaction product, a fraction of the gaseous part of said effluent being recycled to the reaction zone, the liquid part of said effluent being fed back into the fractionation zone, generally close to the point where it was drawn off. The hydroisomerization reaction of but-1-ene into but-2-enes is not mentioned.